This invention relates to electronic cable termination. More particularly, this invention relates to multi-protocol cable-termination circuits.
Electronic devices communicate with each other by transmitting and receiving signals (e.g., data and control) via signal lines or cables. Signal cables should be properly terminated to dissipate the energy of the signal pulse, and to reduce the undesirable transmission line effects of signal reflection or "ringing." Signal cables are usually terminated with a resistive termination circuit typically connected in parallel with driver and receiver circuits. To determine the proper configuration and impedance values of a termination circuit, particular electrical information, such as, for example, transmitted signal voltage levels, signal cable impedance, and the internal impedance of the driver and receiver circuits should be known.
Electrical interface standards were developed to standardize, among other things, cable terminations between communicating devices. These interface standards provide electrical specifications, known as protocols, that specify termination configurations and impedance values. Over the years, however, many standards evolved to cover either broad areas of information transmission or unique requirements in specific applications. For example, in the United States, the Electronics Industries Association (EIA) developed a number of different standards, such as, for example, RS-232, RS-422, and RS-423. Similarly in Europe, the Comite Consultif Internationale Telegraphique et Telephone (International Consultative Committee for Telegraph and Telephone, or CCITT) also developed a number of different standards, such as, for example, V.10, V.11, V.28, and V.35. For the most part, the EIA and CCITT standards are compatible. For example, RS-422 is compatible with V.11, RS-232 is compatible with V.28, and RS-423 is compatible with V.10. These interface standards have been accepted generally by most manufacturers of electronic data transmission and business equipment.
Thus, if a device communicates with other devices that each adhere to the same interface standard, a termination circuit in conformance with that standard can be readily ascertained and used. However, if a device communicates with other devices, two or more of which adhere to different interface standards, several termination circuits, each in conformance with one of the interface standards, are typically required.
Commonly, multiple cable connectors or termination modules are provided that each conform to a different interface standard. Unfortunately, these connectors or modules must be manually reconnected each time the interface standard changes. Moreover, access to these connectors or modules should be maintained to permit reconnecting. Custom cables, each with a particular termination built into the cable head, are also known, but they too unfortunately require manual reconnecting. Furthermore, such custom cables are often expensive. Separate connectors, termination modules, and custom cables, moreover, require manufacturers to stock and maintain large inventories to satisfy the different termination standards, thereby further increasing costs.
External switching resistors with electromechanical relay circuits that can switch terminations in response to received signals are also known. These devices eliminate the time-consuming and cumbersome task of manually changing cable connections. However, such devices are often expensive and physically large, thus increasing costs and requiring considerable space--space that either is not readily available or cost-effective for such use. Separate terminations built onto a circuit board with a custom cable that automatically routes the signals to the appropriate termination are also known. But these too require considerable space to accommodate the large connectors that are needed. In sum, known cable terminations are impractical because they are either expensive, cumbersome, time-consuming, or physically large.
Designing an integrated multi-protocol cable-termination circuit that is advantageous in comparison to the known techniques described above presents a problem of how to maintain a particular termination configuration when voltages on the input terminals exceed supply voltages or when power to the cable-termination circuit is off. For example, if MOSFETs (metal-oxide-semiconductor field effect transistors) are used to switch termination configurations, non-conducting MOSFETs may incorrectly turn ON (i.e., conduct) when the input signal voltage exceeds the supply voltage, because the inherent well and substrate diodes in those non-conducting MOSFETs may forward bias and thus conduct. Input signal voltages can exceed supply voltages when, for example, the V.28/RS-232 electrical interface standard is adhered to, because input voltage swings of +/-15 volts are likely, while MOSFET supply voltages are usually in the +/-5 volt range.
Similarly, non-conducting MOSFETs may also turn ON incorrectly when power to the cable-termination circuit is off while voltages at the input terminals are of such a magnitude that either the gate-to-drain or gate-to-source voltage forces a MOSFET into conduction, or the inherent well and substrate diodes forward bias, resulting in conduction.
In view of the foregoing, it would be desirable to provide an integrated multi-protocol cable-termination circuit that is physically small, requires little access space, and can be selectably automatically configured to terminate a signal cable in conformance with one of a plurality of electrical interface standards.
It would also be desirable to provide an integrated multi-protocol cable-termination circuit that maintains a selected termination configuration when input voltages exceed supply voltages or when power to the termination circuit is off.
It would further be desirable to provide an integrated selectable cable terminator for terminating multiple cables that includes a plurality of multi-protocol cable-termination circuits that can be selectably automatically configured to conform to one of a plurality of electrical interface standards.